Recently, optical communication systems have employed a wavelength-division multiplexing (WDM) signal processing method. This has dramatically increased transmission capacity of the optical communication systems.
In this case, various optical functional devices may be required to provide advanced functions for the optical signal processing. A bending optical waveguide is used for changing a direction of light propagation in order to achieve smaller-size and higher-density of optical functional elements. Such a bending waveguide may be provided in order to prevent light reflection at an end face of an optical functional element.
Excitation of a higher-order mode in an optical functional element, especially the element that includes an optical branching and coupling element and an interferometer significantly influences the element and eventually deteriorates performance of the optical communication system. The smaller the size of the optical functional element (optical integrated device), the more likely the excited higher-order mode further excites a leakage mode and could cause negative effects.
Accordingly, various methods are proposed to remove an excited higher-order mode.
A method (first method) is proposed in which an S-shaped bending region is provided in one waveguide side of an optical branching and coupling element to remove an excited higher-order mode by making light radiate at the S-shaped bending region.
Another method (second method) is proposed in which a width of one waveguide side of an optical branching and coupling element is made narrower to remove a higher-order mode, and a width of a bending waveguide of a branching part (coupling part) is made wider to suppress a bending loss.
Furthermore, a third method is proposed in which a filter for removing a higher-order mode is provided in one waveguide side of an optical branching and coupling element. For example, when a 1×1 multimode interference (MMI) coupler is provided as a filter for removing a higher-order mode, a fundamental mode passes with little or no excessive loss, however the higher-order mode is subject to substantial loss and may not pass, thus higher-order mode may be selectively removed. The third method, however, increases the number of elements, thereby enlarging the device size. Moreover, an insertion loss due to the filter for removing the higher-order mode and wavelength dependence of the filter for the removing higher-order mode itself is additionally caused, which may influence device characteristics.
Related techniques are disclosed in Japanese Laid-open Patent Publications No. hei. 06-67047, and No. 2006-278770.
Each of the foregoing methods assumes that the higher-order mode is excited and removes a higher-order mode so that the excited higher-order mode may not exert negative influence.
However, it is desirable that a higher-order mode is not excited.
On the other hand, it is found that providing an S-shaped bending region or a bending waveguide excites a higher-order mode at the bending region of the S-shaped bending region or the bending waveguide.
Therefore, it is desirable that excitation of a higher-order mode itself in a bending waveguide be prevented.